The present invention relates to a transceiver for a portable radio communication apparatus and in particular to a transceiver using direct conversion in the receiver.
A general trend in portable communication apparatus is the reduction in volume weight and power consumption of such apparatus. This had lead to efforts towards reducing the number of elements and devices necessary to perform the functions associated with portable communication apparatus. One approach has been to reduce the number of stages in the radio frequency front end by converting a received radio frequency carrier signal down to a DC intermediate frequency (zero IF) in a single step. This is known as direct conversion.
To carry out direct conversion, a local oscillator signal (LO) having the same frequency as the radio frequency carrier signal (ie. the LO is xe2x80x9con-channelxe2x80x9d) is mixed with the carrier signal in a suitable non-linear device such as a mixer diode. The output of the mixer contains the sum and difference of the LO and the carrier signal. Thus a mixer product exists at twice the carrier signal, and also at DC (zero Hz). The high frequency mixer products can be filtered by a suitable low pass filter. Once the radio frequency carrier signal has been down-converted, the modulating signal may be de-modulated using an appropriate demodulator, e.g. an I/Q demodulator for an I/Q modulating signal.
In the field of radio telephony, particularly cellular telephony, use of a direct conversion receiver is not without certain drawbacks.
Radio telephones operating in the same cell of a cellular telephone network typically receive control signals on the same signal frequency as each other. This is necessary since such control signals are often so-called xe2x80x9cbroadcastxe2x80x9d control signals such as paging signals which need to be received by all the telephones operating in the same cell. Signals received by radio telephones can be of very low power, e.g. below xe2x88x92100 dBm and the sensitivity of the receivers is adapted accordingly.
Against the requirement for radio telephones in the same cell to be tuned to common frequencies, the introduction of direct conversion receivers with their strong on-board LO signals presents the problem of interference. More specifically, spurious LO leakage from the receiver into the antenna can cause in-band interference with other nearby receivers tuned to the same channel. Therefore, such a receiver would require a very high level of isolation between the local oscillator and the antenna in order to avoid swamping or saturating receivers of any nearby radio telephones. Such high levels of isolation are very difficult to achieve due to stray capacitances directly coupling local oscillator energy into the antenna. This is the case even when conventional mixers such as a Gilbert cell is used.
Previous methods addressing the problem of LO leakage included:
providing increased reverse-isolation in the RF path to the antenna, for example introducing multi-stage RF amplifiers,
inserting an isolator,
reducing the local oscillator drive power to very low levels.
However, these methods were seen to have certain shortcomings, in particular, they were found to be detrimental to dynamic range and degrading to sensitivity, as well as being expensive to implement.
The present inventors devised a solution to the problem LO interference by providing a receiver which uses sub-harmonic mixing whereby, the received signal can be down-converted without employing a local oscillator at the carrier frequency of the received signal. Accordingly, the local oscillator signal frequency is far removed from received signal frequency, resulting in the mixer providing very high local oscillator to received signal isolation. This is because leakage of local oscillator radiation from the receiver to the antenna has negligible effect on other receivers in close proximity receiving on the same channel as the received signal. Thus, less shielding is necessary to inhibit interference signals generated by the local oscillator leaking out and interfering with adjacent apparatus than for a local oscillator near or at the radio frequency signal. This invention is disclosed in GB patent publication number 2 332 796 A.
Whilst running the local oscillator at a fraction of the received signal frequency is advantageous in terms of the receiver it imposes new criteria on the transmitter design.
In cellular radiotelephone transceiver design it is standard to couple the local oscillator used in the receiver to the transmitter. In other words, the transmitter runs off the same local oscillator as that used by the receiver, i.e. it is xe2x80x98slavedxe2x80x99 to the receiver LO. This ensures that the correct duplex spacing is maintained between the transmitter and the receiver, that is to say the transmitter operates at a frequency level which is a fixed, predetermined and constant frequency spacing from the receiver channel. In GSM the duplex spacing is 45 MHz in 900 MHz band, and 75 MHz in 1.8 GHz band.
In conventional direct conversion transceivers the requirement for the transmitter to run from the same local oscillator as the receiver is met because the local oscillator is on-channel, i.e. it runs at the carrier frequency. However, in a direct conversion receiver using a local oscillator at a subharmonic of the reference frequency, it is no longer possible directly to use the local oscillator in the transmitter design.
Therefore, the present inventors set about to solve the problem of providing a transceiver in which a direct conversion receiver operating from a subharmonic local oscillator could be xe2x80x98slavedxe2x80x99 to the transmitter. One proposal was to mix the local oscillator with a further introduced (ultra high frequency) synthesiser. This approach was however found to be unsatisfactory because of complexity, and because the introduction of the strong local oscillator was detrimental to performance.
Accordingly, one aspect of the present invention provides a transceiver for a portable radio communication apparatus comprising a receiver for receiving a signal at a received frequency and including a local oscillator, a transmitter being operable to transmit at a transmission frequency, said transmission frequency being offset from, and aligned to, the received frequency by a predetermined frequency spacing, the transmitter including a mixer having a first port for inputting said transmission frequency signal and a second port for inputting the local oscillator signal, and comprising means for rectifying the input local oscillator signal to provide a conductance waveform at a multiple of the local oscillator signal and means for mixing said transmission frequency signal with the conductance waveform at said multiple of the local oscillator signal frequency for down-converting the transmission frequency signal to a transmitter intermediate frequency signal.
By means of the invention, the mixer allows mixing of the receiver local oscillator with the transmission frequency without the need to introduce a further VHF oscillator, thereby substantially reducing the risk of high frequency local oscillator leakage, and the associated additional cost, power consumption and complexity (of the extra synthesiser).
In a first embodiment of the present invention, the rectifying means and the coupling means comprise a balanced anti-phase output transformer and switching means, wherein a signal output from the transformer drives the switching means at a multiple of the local oscillator frequency.
The balanced anti-phase output transformer further improves isolation between the input (radio frequency) port and the local oscillator port. Optionally, the transformer may be a strip line, slot-line, co-planar microstrip and/or microstrip balanced to unbalanced transformer. As an alternative to the transformer, a ceramic type balanced phase splitter could also be used.
Preferably, the switching means comprises an anti-parallel diode pair, and respective diodes are electrically coupled to respective arms of the balanced output of the transformer, and suitably the diodes are Schottky diodes.
Advantageously, the switching means comprises a further anti-parallel diode pair, and respective diodes of the further anti-parallel pair are reversibly electrically coupled to respective arms of the balanced output of the transformer relative to the said anti-parallel diode pair. Such a dual anti-parallel diode pair configuration provides a higher balance between arms of the switching means than achievable with a single anti-parallel diode pair. This further improves isolation between the radio frequency and local oscillator ports and reduction in local oscillator second harmonic.
In a second embodiment of the present invention, the rectifying means and the coupling means comprise a phase splitter and a plurality of transistors arranged in a network, wherein a signal output from the driver transistor drives the transistor network at a multiple of the local oscillator frequency.
The use of transistors in the second embodiment advantageously provides for the integration of the embodiment into an integrated circuit.
Preferably, the phase splitter provides anti-phase local oscillator drive signals to the plurality of transistors arranged in a network, and the driver transistor may comprise a unipolar transistor, conveniently an FET transistor, while the plurality of transistors arranged in a network may comprise bipolar transistors in a bridge configuration, or FET""s.
For this application of direct conversion receiver the local oscillator is half the received frequency.
An advantageous feature of the mixers disclosed herein is that they are bi-directional.